Grass lawns are now commonly produced by growing sod in large fields at special sod farms. The fields are prepared in a similar manner to vegetable farming with special attention given to the final grade. The grass seed is sown using a mechanical seeder and is then irrigated for germination. Once germinated, the sod is systematically irrigated, mowed and weeded for a minimum of five months and up to a year or more before a harvestable crop is established. The sod is then mechanically cut into strips containing approximately 5/8ths of an inch of top soil. The strips are then folded or rolled and finally stacked onto pallets for shipping via truck.
Barrier grown sod has evolved into a viable alternative to field grown sod. Using this method, sod is produced by placing a thin plastic barrier over the field and laying a mixture of growth media and seed over the plastic. The plastic acts as a barrier to vertical root development, forcing lateral growth which reduces production time. Because the organic growth media is significantly lighter than soil, the product has the additional potential of reducing trucking and installation costs. See, e.g., U.S. Pat. No. 4,720,935 to Rogers et al. Finally barrier grown sod does not result in the reduction of top soil.
The production of barrier grown sod requires precise control of the root zone environment. There are serious agronomic and logistical issues that must be resolved in order to effectively produce barrier grown sod on a large scale. Because barrier grown sod has a root zone hundreds of times smaller than traditional field grown sod, conditions such as salinity, oxygenation, moisture content, and fertility must be carefully controlled for healthy sod production. Furthermore, local conditions such as water quality and growing media complicate the growers' ability to maintain a balanced and healthy environment. A change in the system to resolve an imbalance in any one of the factors will affect the remaining factors. For example, frequent irrigation to maintain optimum moisture content can result in salinity buildup. Alternatively, the use of a fine particle media for moisture retention reduces the media's ability to hold oxygen necessary for healthy root development,
Therefore, when producing barrier grown sod, it is essential to regulate the drainage of excess water and to leach out salts that may result from fertilizer breakdown. The common approach used to resolve the drainage requirement is to perforate the plastic barrier. Chamoulaud, U.S. Pat. No. 4,232,481 uses such a perforated plastic sheet on a substantially flat surface to allow for drainage. The primary deficiency of this approach however, is that the soil beneath the barrier becomes wet and muddy from drainage water which seeps through the holes. Unless the producer has access to high drainage sandy soil, the fields will soften and eventually be unable to adequately support the weight of a mower or other equipment.
Others have used a plastic sheet impervious to moisture. For example, U.K. Patent Application 2,200828A to Buller describes such use of an impervious plastic sheet on level and flat ground with edging batons combined with drainage channels between the row mounds to drain away excess water. The edging batons and flat growing surface however, limit its drainage potential. Therefor, this approach can only be successfully applied in areas with access to low salinity irrigation water and/or significant rainfall.
Accordingly, there exists a need for a commercial barrier grown sod production method that allows for good drainage control. There is a further need for a method which permits watering the sod, but ensures that the soil beneath the plastic barrier remains substantially dry and solid, in order to support working vehicles. There is also a need for a method that effectively balances salinity, oxygenation, moisture content and fertility, even when using less than ideal irrigation water and growing media. The present invention fulfills these needs and provides further related advantages.